That... seems pretty unsafe. If I'm taking a backup, I definitely would avoid resizing it or making any modifications to it during the backup process. What if the resize fails and is the reason you need to restore from backup in the first place?

I guess it's a handy hack in use cases like yours, or if the backup is a convenience, but it's important to understand the risks and whether you're better off with filesystem level tools.

You'd probably be better off with dd if=/dev/zero of=file.zero to zero out empty space, dd copy the whole drive, then compress the copy. I wouldn't fuck around with partitions on something I want to back up

Reposted from a server fault thread , author plasmapotential. note fdisk -l -u=cylinders /dev/sdX will output cylinder info if it doesnt by default.

Use dd, with the count option.

In your case you were using fdisk so I will take that approach. Your "sudo fdisk -l "produced:

Disk /dev/sda: 64.0 GB, 64023257088 bytes
255 heads, 63 sectors/track, 7783 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0000e4b5

Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          27      209920   83  Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2              27         525     4000768    5  Extended
Partition 2 does not end on cylinder boundary.
/dev/sda5              27         353     2621440   83  Linux
/dev/sda6             353         405      416768   83  Linux
/dev/sda7             405         490      675840   83  Linux
/dev/sda8             490         525      282624   83  Linux

The two things you should take note of are 1) the unit size, and 2) the "End" column. In your case you have cylinders that are equal to 8225280 Bytes. In the "End" column sda8 terminates at 525 (which is 525[units]16065512 = ~4.3GB)

dd can do a lot of things, such as starting after an offset, or stopping after a specific number of blocks. We will do the latter using the count option in dd. The command would appear as follows:

sudo dd if=/dev/sda of=/your_directory/image_name.iso bs=8225280 count=526

Where -bs is the block size (it is easiest to use the unit that fdisk uses, but any unit will do so long as the count option is declared in these units), and count is the number of units we want to copy (note that we increment the count by 1 to capture the last block).